1. Field of the Invention
The present invention relates to a optical waveguide element that may be employed in, for instance, optical communication and a method for manufacturing the optical waveguide element.
2. Description of the Related Art
Passive light-wave circuits are playing an increasingly crucial role in optical communication systems today. A wave guiding channel type light-wave circuit that achieves a high degree of stability and excellent mass productivity is considered to be an essential component in optical communication systems. In particular, quartz optical waveguides, which make the most of the physical and chemical stability of quartz glass, achieve advantages such as good conformity with optical fibers constituting transmission pathways and have been adopted in applications in PLCs (planar light-wave circuits) having complex and advanced functions to enable control of light phases and interference, and intense research and development efforts have been made in this area.
FIG. 11 illustrates a schematic structure of a optical waveguide element 500 which may be employed in a PLC in the prior art and FIG. 12 illustrates the process implemented to manufacture quartz optical waveguides in the prior art. A lower clad layer 504 and a core layer 506 having SiO2 as their main constituents are formed on an Si substrate 502 through a method Which uses to its advantage a gas-phase reaction such as the CVD (chemical vapor deposition) method or the FHD (flame hydrolysis deposition) method. The lower clad layer 504 and the core layer 506 are differentiated from each other by forming them with source gases having different compositions.
Next, unnecessary portions of the core layer 506 are removed through dry etching such as RIE (reactive ion etching) or RIBE (reactive ion beam etching). As a result, the remaining core layer 506, left in ridges, forms core portions 508.
Lastly, an upper clad layer 510 having SiO2 as its main constituent is formed so as to cover the core portions 508 through a method which uses a gas-phase reaction to advantage. The optical waveguide element 500 in the prior art is thus obtained. It is to be noted that in this optical waveguide element 500, quartz optical waveguides constituted of the lower clad layer 504, the core portions 508 and the upper clad layer 510 are formed.
However, as explained above, only one core layer is formed on the substrate constituted of Si or the like at the optical waveguide element in the prior art. Consequently, the optical waveguide element in the prior art is subject to a restriction under-which the light-wave circuits must be formed essentially within a single plane. The number of optical elements mounted at a single chip has been increasing to support even more advanced and diversified functions that PLCs must fulfill in recent years. The optical waveguide element subject to the restriction described above can only keep up with this trend by increasing the chip size, which in turn, leads to an increase in production costs.
As a solution to the common problem of PLCs in the prior art described above, Japanese Unexamined Patent Publication No. 1995/20344, for instance, discloses a method for forming a multilayer light-wave circuit substrate by alternately laminating a light-wave circuit substrate with optical waveguide circuit patterns formed therein and spacers.
However, since a spacer and a substrate are present between light-wave circuit layers that are next to each other, interference or coupling cannot be achieved between light-wave circuits formed in the different light-wave circuit layers through the method disclosed in the publication above. As a result, light-wave circuits must be formed within a single light-wave circuit layer if it is necessary to achieve interference or coupling between the light-wave circuits, just as in standard PLCs.
The present invention has been completed by addressing the problems discussed above and other problems of the optical waveguide element in the prior art.
Accordingly, the present invention as disclosed in claim 1 provides a optical waveguide element having n light-wave circuit layers each constituted of a core portion and a clad layer covering the core portion that are sequentially laminated to achieve a multilayer structure, with n representing an integer equal to or larger than 2.
According to the present invention as disclosed in claim 1, at least one optical waveguide with a core portion constituting a transmission path for light is formed within each light-wave circuit layer. Three-dimensional placement of a group of optical waveguides is achieved through the multilayer structure of the light-wave circuit layers. Thus, according to the present invention as disclosed in claim 1, the number of optical waveguides per unit area can be increased.
In addition, according to the present invention as disclosed in claim 1, at least two light-wave circuit layers are sequentially laminated without a substrate or a spacer present between them. Consequently, it is possible to achieve an interaction such as interference or coupling, of guided light-waves in the optical waveguides formed in different light-wave circuit layers. In addition, light-wave circuits can be formed three-dimensionally.
As explained above, according to the present invention a much higher degree of freedom is afforded in the formation of light-wave circuits through the increase in the number of optical waveguides per unit area and the three-dimensional formation of light-wave circuits. As a result, various types of light-wave circuits can be formed without having to take up a larger mounting area for the optical waveguide elements. This is expected to contribute to higher integration and further miniaturization of optical apparatuses.
It is to be noted that the core portions in the individual light-wave circuit layers may be formed at various positions according to the invention. For instance, a core portion may be positioned so that it is in complete contact with the surface constituting the boundary with the adjacent light-wave circuit layer, may be positioned so that it comes into partial contact with the surface constituting the boundary with the adjacent light-wave circuit layer at, at least, one point or may be positioned so that it is completely isolated from the adjacent light-wave circuit layer.
According to a feature of the present invention, an optical waveguide element having formed therein at least one optical coupler astride two or more light-wave circuit layers contiguous to each other in a multilayer structure is provided. This achieves a three-dimensional formation of light-wave circuits over a plurality of light-wave circuit layers. The optical coupler may be constituted of an optical coupler with a uniform distance between the optical waveguides at the coupled area such as a directional optical coupler or a parallel three-wave guiding channel directional optical coupler, or it may be constituted of a directional optical coupler with variable distances between the optical waveguides, for instance.
In addition, the optical coupler may be formed through any of various combinations of core portion groups in the contiguous light-wave circuit layers. According to the present invention, each of the contiguous light-wave circuit layers may include at least one core portion to constitute the optical coupler, the core portions constituting the optical coupler may be included only in every other light-wave circuit layer or the core portions constituting the optical coupler may be completely randomly provided among the contiguous light-wave circuit layers. It is to be noted that the core portions constituting the optical coupler in the structure fulfill a function of guiding light energy, a function of relay coupling two other core portions or a function achieving a combination of these functions.
According to a further feature of the present invention an optical waveguide element having formed therein an optical coupler astride two contiguous light-wave circuit layers in a multilayer structure is provided. The optical coupler is constituted of at least two core portions. Accordingly, at least one core portion is formed in each of the two ligh t-wave circuit layers to constitute the optical coupler . In addition, since the optical coupler is formed astride the two light-wave circuit layers, there is at least one pair of core portions coupled with each other over the boundary of the light-wave circuit layers among the two or more core portions constituting the optical coupler. According to the present invention as disclosed in claim 3, the coefficient of coupling of the optical coupler can be adjusted by controlling the distance between the pair of core portions.
According to still a further feature of the present invention, an optical waveguide element having formed therein an optical coupler astride three contiguous light-wave circuit layers in a multilayer structure which achieves coupling of core portions in the light-wave circuit layers at the two ends via a core portion in the intermediate light-wave circuit layer, is provided. It is to be noted that this embodiment of the invention may assume a structure in which the core portion in the intermediate light-wave circuit layer is terminated within the intermediate light-wave circuit. layer and, therefore, does not fulfill the light-wave guiding function. Or, the core portion in the intermediate light-wave circuit layer may be left unterminated so that it, too, fulfills a light-wave guiding function, instead.
In addition, according to the present invention, an optical waveguide element assuming a structure with core portions formed as channels is provided. This embodiment of the invention makes it possible to form a planar light-wave circuit with, at least, one core portion formed as a channel in each light-wave circuit layer. Thus, light-wave circuits can be formed both along the direction of lamination and within the individual light-wave circuit layers so that a optical waveguide element which allows a complex and threedimensional formation of light-wave circuits is provided. It is to be noted that in this embodiment of the invention , the core portion channels may assume any of various shapes such as a curved shape, a linear shape or a branching shape.
According to another feature of the present invention, an optical waveguide element with, at least, either the core portions or the clad layers constituted mainly of SiO2 is provided. The resulting structure allows easy matching of the optical characteristics of optical waveguides constituted of core portions and clad layers and the optical characteristics of optical fibers that are normally used as optical signal transmission lines. Thus, the structure disclosed achieves an optical waveguide element that can be easily connected with an optical fiber while manifesting only a small degree of insertion loss in the optical communication system.
The present invention, as disclosed, provides for a further feature in which an optical waveguide element having a substrate constituted of Si which supports n light-wave circuit layers. The structure disclosed may be effectively adopted when core portions and clad layers have SiO2 as their main constituent, e.g., when the core portions and clad portions constitute quartz optical waveguides.
The invention also relates to an optical waveguide element manufacturing method that includes a first step in which a first clad layer is formed, a second step in which a kth core portion is formed on a kth clad layer, a third step in which a (k+1)th clad layer is formed to cover the kth core portion and a fourth step in which the second step and the third step are alternately repeated n times. Through the use of the method according to the invention an optical waveguide element according to the present invention in which the core layer in each light-wave circuit layer is formed in a state in which the core layer is in complete contact with the surface constituting the boundary with an adjacent light-wave circuit layer can be formed. xe2x80x9ckxe2x80x9d represents an integer that is equal to or larger than 1 and equal to or smaller than n.
According to the method of the present invention, in which the core layers are sequentially formed with clad layers enclosed between them, the distance between the individual core layers is controlled in conformance to the thickness of the clad layers. The clad layers may be formed through any of various methods including the CVD method the FHD method, the sputtering method, the vacuum deposition method and the epitaxial method. Any of these formation methods achieves a higher degree of process accuracy compared to photolithography and etching performed for core portion patterning. Thus, the thickness of the clad layers can be controlled with a higher degree of accuracy compared to the accuracy of the core portion patterning.
As described above, according to the method of the present invention, the distances between the core portions are controlled with a high degree of accuracy to achieve the interaction of guided light-waves in the core portions as intended in design, thereby achieving an improvement in the yield of the optical waveguide element according to the present invention.
In addition, according to a feature of the method invention, an optical waveguide element manufacturing method in which the kth clad layer is formed through the CVD method is provided. According to the present method invention, the distance between the core portions can be controlled with an extremely high degree of accuracy by taking advantage of the accurate film thickness control that the CVD method achieves. It is to be noted that while there are various CVD methods such as a normal temperature CVD method, a vacuum CVD method, a plasma CVD method and a laser CVD method in the known art, any one of such CVD methods may be employed in the method invention.
A further feature of the invention provides an optical waveguide element manufacturing method in which the third step comprises a process in which a source material layer from which the kth core portion is to be constituted is formed on the kth clad layer and a process in which unnecessary portions of the source material layer are removed through reactive ion etching to form the kth core portion. According to this feature of the present invention, core portions can be formed as channels achieving a specific pattern. Thus, a highly complex light-wave circuit can be achieved through the interaction of the core portions along the direction of the lamination and through the core portion pattern achieved within a plane perpendicular to the direction of the lamination.